System and method of communication via embedded modulation

ABSTRACT

A communication system includes a master transceiver in communication with one or more transceivers, for example, a first tributary transceiver and a second tributary transceiver over a communication medium. The first tributary transceiver uses a primary modulation method for communication while the second tributary transceiver uses a secondary or embedded modulation method for communication. The master transceiver and tributary transceivers each include a processor, memory, and control logic for controlling their operation. While the primary modulation method is normally used for transmissions on the communication medium, the master transceiver can communicate with the second tributary transceiver by notifying the first tributary transceiver that the primary modulation method is being temporarily replaced by the secondary or embedded modulation method. The master transceiver can then exchange information with the second tributary transceiver while the first tributary transceiver ignores any secondary modulation transmissions. In the meantime, the first tributary transceiver conditions itself to look for a trailing sequence from the master transceiver indicating that communication with the second tributary transceiver is complete. When the master transceiver transmits the trailing sequence using the primary modulation method, the first tributary transceiver conditions itself to look for primary modulation transmissions while the second tributary transceiver conditions itself to ignore primary modulation transmissions.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of the filing dateof U.S. Provisional Application No. 60/067,562, entitled “EmbeddedModulations,” filed Dec. 5, 1997, which is incorporated by reference asif set forth in full hereinbelow.

TECHNICAL FIELD

The present invention relates generally to the fields of datacommunications and modulator/demodulators (modems), and, moreparticularly, to a data communications system in which a plurality ofmodulation methods are used to facilitate communication among aplurality of modem types.

BACKGROUND OF THE INVENTION

In existing data communications systems, a transmitter and receivermodem pair can successfully communicate only when the modems arecompatible at the physical layer. That is, the modems must usecompatible modulation methods. This requirement is generally trueregardless of the network topology. For example, point-to-point, dial-upmodems operate in either the industry standard V.34 mode or the industrystandard V.22 mode. Similarly, in a multipoint architecture, all modemsoperate, for example, in the industry standard V.27bis mode. While themodems may be capable of using several different modulation methods, asingle common modulation is negotiated at the beginning of a datasession to be used throughout the duration of the session. Should itbecome necessary to change modulation methods, the existing data sessionis torn down, and a new session is negotiated using the new modulationmethod. Clearly, tearing down an existing data session causes asignificant disruption in communication between the two modems.

As discussed in the foregoing, communication between modems is generallyunsuccessful unless a common modulation method is used. In apoint-to-point network architecture, if a modem attempts to establish acommunication session with an incompatible modem, one or both of themodems will make several attempts to establish the communication linkuntil giving up after a timeout period has expired or the maximum numberof retry attempts has been reached. Essentially, communication on thelink is impossible without replacing one of the modems such that theresulting modem pair uses a common modulation method.

In a multipoint architecture, a single central, or “master,” modemcommunicates with two or more tributary or “trib” modems using a singlemodulation method. If one or more of the trib modems are not compatiblewith the modulation method used by the master, those tribs will beunable to receive communications from the master. Moreover, repeatedattempts by the master to communicate with the incompatible trib(s) willdisturb communications with compatible trib(s) due to time wasted inmaking the futile communication attempts.

Thus, communication systems comprised of both high performance and lowor moderate performance applications can be very cost inefficient toconstruct. For example, some applications (e.g., internet access)require high performance modulation, such as quadrature amplitudemodulation (QAM), carrier amplitude and phase (CAP) modulation, ordiscrete multitone (DMT) modulation, while other applications (e.g.,power monitoring and control) require only modest data rates andtherefore a low performance modulation method. All users in the systemwill generally have to be equipped with a high performance modem toensure modulation compatibility. These state of the art modems are thenrun at their lowest data rates for those applications that requirerelatively low data throughput performance. The replacement ofinexpensive modems with much more expensive state of the art devices dueto modulation compatibility imposes a substantial cost that isunnecessary in terms of the service and performance to be delivered tothe end user.

Accordingly, what is sought, and what is not believed to be provided bythe prior art, is a system and method of communication in which multiplemodulation methods are used to facilitate communication among aplurality of modems in a network, which have heretofore beenincompatible.

SUMMARY OF THE INVENTION

The present invention is generally directed to a system and method ofcommunication between a master transceiver and a plurality of tributarytransceivers in a multipoint communication system in which the tributarytransceivers use different types of modulation methods. Broadly stated,the communication system includes a master transceiver in communicationwith a first tributary transceiver and a second tributary transceiverover a communication medium. The first tributary transceiver uses aprimary modulation method for communication while the second tributarytransceiver uses a secondary or embedded modulation method forcommunication. The master transceiver and tributary transceivers eachinclude a processor, memory, and control logic for controlling theiroperation. While the primary modulation method is normally used fortransmissions on the communication medium, the master transceiver cancommunicate with the second tributary transceiver by notifying the firsttributary transceiver that the primary modulation method is beingtemporarily replaced by the secondary or embedded modulation method. Themaster transceiver can then exchange information with the secondtributary transceiver while the first tributary transceiver ignores anysecondary modulation transmissions. In the meantime, the first tributarytransceiver conditions itself to look for a trailing sequence from themaster transceiver indicating that communication with the secondtributary transceiver is complete. When the master transceiver transmitsthe trailing sequence using the primary modulation method, the firsttributary transceiver conditions itself to look for primary modulationtransmissions while the second tributary transceiver conditions itselfto ignore primary modulation transmissions.

The present invention has many advantages, a few of which are delineatedhereafter as merely examples.

One advantage of the present invention is that it provides to the use ofa plurality of modem modulation methods on the same communicationmedium.

Another advantage of the present invention is that a master transceivercan communicate seamlessly with tributary transceivers or modems usingincompatible modulation methods.

Other features and advantages of the present invention will becomeapparent to one with skill in the art upon examination of the followingdrawings and detailed description. It is intended that all suchadditional features and advantages be included herein within the scopeof the present invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention can be better understood with reference to thefollowing drawings. The components and representations in the drawingsare not necessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present invention. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a block diagram of a prior art multipoint communication systemincluding a master transceiver and a plurality of tributarytransceivers;

FIG. 2 is a ladder diagram illustrating the operation of the multipointcommunication system of FIG. 1;

FIG. 3 is a block diagram of a master transceiver and tributarytransceiver for use in the multipoint communication system of FIG. 1 inaccordance with the principles of the present invention;

FIG. 4 is a block diagram of a multipoint communication system includingthe master transceiver and a plurality of tributary transceivers of thetype illustrated in FIG. 3;

FIG. 5 is a ladder diagram illustrating the operation of the multipointcommunication system of FIG. 4;

FIG. 6 is a state diagram for a tributary transceiver of FIGS. 3-5 usinga secondary modulation method in accordance with the principles of thepresent invention; and

FIG. 7 is a state diagram for a tributary transceiver of FIGS. 3-5 usinga primary modulation method in accordance with the principles of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

While the invention is susceptible to various modifications andalternative forms, a specific embodiment thereof is shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that there is no intent to limit theinvention to the particular form disclosed, but on the contrary, theinvention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defmed by theclaims.

With reference to FIG. 1, a prior art multipoint communication system 22is shown to comprise a master modem or transceiver 24, whichcommunicates with a plurality of tributary modems (tribs) ortransceivers 26—26 over communication medium 28. Note that all tribs26—26 are identical in that they share a common modulation method withthe master transceiver 24. Thus, before any communication can begin inmultipoint system 22, the master transceiver and the tribs 26—26 mustagree on a common modulation method. If a common modulation method isfound, the master transceiver 24 and a single trib 26 will then exchangesequences of signals that are particular subsets of all signals that canbe communicated via the agreed upon common modulation method. Thesesequences are commonly referred to as training signals and can be usedfor the following purposes: 1) to confirm that the common modulationmethod is available, 2) to establish received signal level compensation,3) to establish time recovery and/or carrier recovery, 4) to permitchannel equalization and/or echo cancellation, 5) to exchange parametersfor optimizing performance and/or to select optional features, and 6) toconfirm agreement with regard to the foregoing purposes prior toentering into data communication mode between the users. In a multipointsystem, the address of the trib with which the master is establishingcommunication is also transmitted during the training interval. At theend of a data session a communicating pair of modems will typicallyexchange a sequence of signals known as trailing signals for the purposeof reliably stopping the session and confirming that the session hasbeen stopped. In a multipoint system, failure to detect the end of asession will delay or disrupt a subsequent session.

Referring now to FIG. 2, an exemplary multipoint communication sessionis illustrated through use of a ladder diagram. This system uses polledmultipoint communication protocol. That is, a master controls theinitiation of its own transmission to the tribs and permits transmissionfrom a trib only when that trib has been selected. At the beginning ofthe session, the master transceiver 24 establishes a common modulationas indicated by sequence 32 that is used by both the master 24 and thetribs 26 a, 26 b for communication. Once the modulation scheme isestablished among the modems in the multipoint system, The mastertransceiver 24 transmits a training sequence 34 that includes theaddress of the trib that the master seeks to communicate with. In thiscase, the training sequence 34 includes the address of trib 26 a. As aresult, trib 26 b ignores training sequence 34. After completion of thetraining sequence 34, master transceiver 24 transmits data 36 to trib 26a followed by trailing sequence 38, which signifies the end of thecommunication session. Note that trib 26 b ignores data 36 and trailingsequence 38 as it was not requested for communication during trainingsequence 34.

At the end of trailing sequence 38, trib 26 a transmits trainingsequence 42 to initiate a communication session with master transceiver24. Because master transceiver 24 selected trib 26 a for communicationas part of training sequence 34, trib 26 a is the only modem that willreturn a transmission. Thus, trib 26 a transmits data 44 destined formaster transceiver 24 followed by trailing sequence 46 to terminate thecommunication session.

The foregoing procedure is repeated except master transceiver identifiestrib 26 b in training sequence 48. In this case, trib 26 a ignores thetraining sequence 48 and the subsequent transmission of data 52 andtrailing sequence 54 because it does not recognize its address intraining sequence 48. Master transceiver 24 transmits data 52 to trib 26b followed by trailing sequence 54 to terminate the communicationsession. To send information back to master transceiver 24, trib 26 btransmits training sequence 56 to establish a communication session.Master transceiver 24 is conditioned to expect data only from trib 26 bbecause trib 26 b was selected as part of training sequence 48. Trib 26b transmits data 58 to master transceiver 24 terminated by trailingsequence 62.

The foregoing discussion is based on a two-wire, half-duplex multipointsystem. Nevertheless, it should be understood that the concept isequally applicable to four-wire systems.

Consider the circumstance in which master transceiver 24 and trib 26 bshare a common modulation type A while trib 26 a uses a secondmodulation type B. When master transceiver attempts to establish A as acommon modulation during sequence 32, trib 26 a will not be able tounderstand that communication. Moreover, trib 26 a will not recognizeits own address during training interval 34 and will therefore ignoredata 36 and trailing sequence 38. Master transceiver 24 may time outwaiting for a response from trib 26 a because trib 26 a will nevertransmit training sequence 42, data 44, and trailing sequence 46 due tothe failure of trib 26 a to recognize the communication request(training sequence 34) from master transceiver 24. Thus, if the tribs ina multipoint communication system use a plurality of modulation methods,the overall communication efficiency will be disrupted as specific tribswill be unable to decipher certain transmissions from the mastertransceiver and any unilateral transmission by a trib that has not beenaddressed by the master transceiver will violate the multipointprotocol.

As discussed hereinbefore, however, it is desirable to design amultipoint communication system comprising tribs that use a plurality ofmodulation methods. For example, one moderately priced trib may be usedto communicate at a relatively high data rate for some applications,such as Internet access, while another, lower priced, trib is used tocommunicate at a lower data rate for other applications, such as powermonitoring and control. The needs of these different applications cannotbe efficiently met by a single modulation. While it is possible to usehigh performance tribs running state of the art modulation methods suchas QAM, CAP, or DMT to implement both the high and low data rateapplications, significant cost savings can be achieved if lower costtribs using low performance modulation methods are used to implement thelower data rate applications.

A block diagram of a master transceiver 64 in communication with a trib66 in accordance with the principles of the present invention is shownin FIG. 3. Master transceiver 64 comprises a central processing unit(CPU) 68 in communication with modulator 72, demodulator 74, and memory76. Memory 76 holds software control program 78 and any data necessaryfor the operation of master transceiver 64. Control program 78 includeslogic for implementing a plurality of modulation methods. For purposesof illustration, control program 78 can implement both a type A and atype B modulation through modulator 72 and demodulator 74.

Trib 66 comprises CPU 82 in communication with modulator 84, demodulator86, and memory 88. Memory 88, likewise holds software control program 92and any data necessary for the operation of trib 66. Control programs 78and 92, are executed by CPUs 68 and 82 and provide the control logic forthe processes to be discussed herein. Control program 92 includes logicfor implementing a particular modulation method, which, for purposes ofillustration, is called type X. Inasmuch as master transceiver 64 iscapable of running either a type A or a type B modulation method, type Xrefers to one of those two modulation methods. The master transceiver 64communicates with trib 66 over communication medium 94.

Referring now to FIG. 4, a multipoint communication system 100 is showncomprising a master transceiver 64 along with a plurality of tribs66—66. In this example, two tribs 66 a—66 a run a type A modulationmethod while one trib 66 b runs a type B modulation method. The presentinvention permits a secondary or embedded modulation method (e.g., typeB) to replace the standard modulation method (e.g., type A) after aninitial training sequence. This allows the master transceiver 64 tocommunicate seamlessly with tribs of varying types.

The operation of multipoint communication system 100 will be describedhereafter with reference to the ladder diagram of FIG. 5 and the statediagrams of FIGS. 6 and 7. A communication session between the mastertransceiver 64 and a type B trib 66 b will be discussed first. A statediagram for a type B trib 66 b is shown in FIG. 6. Type B trib 66 b isinitialized in state 102 in which type A modulation transmissions areignored. In the present example, the primary modulation method is typeA, thus, as shown in FIG. 5, master transceiver 64 establishes type A asthe primary modulation in sequence 104. Note that because trib 66 bresponds only to type B modulation transmissions, only the type A tribs66 a—66 a are receptive to transmission sequence 104.

To switch from type A modulation to type B modulation, mastertransceiver 64 transmits a training sequence 106 to type A tribs 66 a inwhich these tribs are notified of an impending change to type Bmodulation. The switch to type B modulation could be limited accordingto a specific time interval or for the communication of a particularquantity of data. After notifying the type A tribs 66 a of the change totype B modulation, master transceiver 64, using type B modulation,transmits data along with an address in sequence 108, which is destinedfor a particular type B trib 66 b. The type B trib 66 b targeted by themaster transceiver 64 will transition to state 112 as shown in FIG. 6upon detecting its own address where it processes the data transmittedin sequence 108.

After completing transmission sequence 108, master transceiver 64transmits a trailing sequence 114 using type A modulation thus notifyingall type A tribs 66 a that type B modulation transmission is complete.If master transceiver 64 has not transmitted a poll request to the typeB trib 66 b in sequence 108, then the type B trib 66 b that was incommunication with the master transceiver 64 will return to state 102after timing out based on the particular time interval defmed for thetype B modulation transmission or transfer of the particular quantity ofdata. Note that the trailing sequence 114 is ineffective in establishingthe termination of a communication session between master transceiver 64and a type B trib 66 b because the trailing sequence is transmittedusing type A modulation.

If, however, master transceiver 64 transmitted a poll request insequence 108, then the type B trib 66 b transitions to state 116 whereit will transmit data, using type B modulation, to master transceiver 64in sequence 118. After completion of this transmission, the type B trib66 b returns to state 102 where type A transmissions are ignored.

With reference to FIG. 5 and FIG. 7, a communication session between themaster transceiver 64 and a type A trib 66 a will now be discussed. Astate diagram for a type A trib 66 a is shown in FIG. 7. A type A trib66 a is initialized in state 122 in which it awaits a type A modulationtraining sequence. If, however, master transceiver transmits a trainingsequence in which the type A tribs 66 a—66 a are notified of a change totype B modulation as indicated by sequence 106, then a transition ismade to state 124 where all type B transmissions are ignored until atype A modulation trailing sequence (e.g., sequence 114) is detected.Upon detecting the type A trailing sequence, a type A trib 66 a returnsto state 122 where it awaits a training sequence.

To initiate a communication session with a type A trib 66 a, mastertransceiver 64 transmits a training sequence 126 in which an address ofa particular type A trib 66 a is identified. The identified type A trib66 a recognizes its own address and transitions to state 128 to receivedata from master transceiver 64 as part of sequence 132.

After completing transmission sequence 132, master transceiver 64transmits a trailing sequence 134 using type A modulation signifying theend of the current communication session. If master transceiver 64 hasnot transmitted a poll request to the type A trib 66 a in sequence 132,then the type A trib 66 a that was in communication with the mastertransceiver 64 will return to state 122 after receiving trailingsequence 134.

If, however, master transceiver 64 transmitted a poll request insequence 132, then the type A trib 66 a transitions to state 136 afterreceiving trailing sequence 134 where it will transmit training sequence138, followed by data sequence 142, and terminated by trailing sequence144 all using type A modulation. After completion of thesetransmissions, the type A trib 66 a returns to state 122 to await thenext type A modulation training sequence by master transceiver 64.

The control programs 78 and 92 of the present invention can beimplemented in hardware, software, firmware, or a combination thereof.In the preferred embodiment(s), the control programs 78 and 92 areimplemented in software or firmware that is stored in a memory and thatis executed by a suitable instruction execution system.

The control programs 78 and 92, which comprise an ordered listing ofexecutable instructions for implementing logical functions, can beembodied in any computer-readable medium for use by or in connectionwith an instruction execution system, apparatus, or device, such as acomputer-based system, processor-containing system, or other system thatcan fetch the instructions from the instruction execution system,apparatus, or device and execute the instructions. In the context ofthis document, a “computer-readable medium” can be any means that cancontain, store, communicate, propagate, or transport the program for useby or in connection with the instruction execution system, apparatus, ordevice. The computer readable medium can be, for example but not limitedto, an electronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, device, or propagation medium. Morespecific examples (a nonexhaustive list) of the computer-readable mediumwould include the following: an electrical connection (electronic)having one or more wires, a portable computer diskette (magnetic), arandom access memory (RAM) (magnetic), a read-only memory (ROM)(magnetic), an erasable programmable read-only memory (EPROM or Flashmemory) (magnetic), an optical fiber (optical), and a portable compactdisc read-only memory (CDROM) (optical). Note that the computer-readablemedium could even be paper or another suitable medium upon which theprogram is printed, as the program can be electronically captured, viafor instance optical scanning of the paper or other medium, thencompiled, interpreted or otherwise processed in a suitable manner ifnecessary, and then stored in a computer memory.

In concluding the detailed description, it should be noted that it willbe obvious to those skilled in the art that many variations andmodifications can be made to the preferred embodiment withoutsubstantially departing from the principles of the present invention.All such variations and modifications are intended to be included hereinwithin the scope of the present invention, as set forth in the followingclaims. Further, in the claims hereafter, the corresponding structures,materials, acts, and equivalents of all means or step plus functionelements are intended to include any structure, material, or acts forperforming the functions with other claimed elements as specificallyclaimed.

Therefore, having thus described the invention, at least the followingis claimed:
 1. A communication system, comprising: a master transceiver;a communication medium; a plurality of tributary transceivers incommunication with said master transceiver over said communicationmedium, a first one of said tributary transceivers using a primarymodulation method for communication over said communication medium and asecond one of said tributary transceivers using a secondary modulationmethod for communication over said communication medium; said mastertransceiver including a processor and a storage medium in communicationwith said processor, said storage medium having program code forexchanging information over said communication medium, said program codecomprising: first logic configured to establish said primary modulationmethod for communication over said communication medium; second logicconfigured to transmit a training sequence to notify said first one ofsaid tributary transceivers of a change from said primary modulationmethod to said secondary modulation method; third logic configured totransmit information to and receive information from said second one ofsaid tributary transceivers using said secondary modulation method;fourth logic configured to transmit a trailing sequence using saidprimary modulation method to notify said first one of said tributarytransceivers of a change from said secondary modulation method to saidprimary modulation method; and fifth logic configured to transmitinformation to and receive information from said first one of saidtributary transceivers using said primary modulation method; said firstone of said tributary transceivers including a processor and a storagemedium in communication with said processor, said storage medium havingprogram code for exchanging information over said communication medium,said program code comprising: sixth logic configured to detect a changefrom said primary modulation method to said secondary modulation method;seventh logic configured to ignore transmissions on said communicationmedium using said secondary modulation method; and eighth logicconfigured to detect a trailing sequence transmitted using said primarymodulation method after detecting a change from said primary modulationmethod to said secondary modulation method; said second one of saidtributary transceivers including a processor and a storage medium incommunication with said processor, said storage medium having programcode for exchanging information over said communication medium, saidprogram code comprising: ninth logic configured to receive informationtransmitted using said secondary modulation method; and tenth logicconfigured to ignore transmissions on said communication medium usingsaid primary modulation method.
 2. The communication system of claim 1,wherein said primary modulation method is selected from the groupconsisting of QAM, CAP, and DMT.
 3. The communication system of claim 1,wherein said secondary modulation is selected from the group consistingof FSK and PAM.
 4. The communication system of claim 1, wherein saidthird logic is configured to transmit a poll request as part of saidinformation.
 5. The communication system of claim 4, wherein saidprogram code in said second one of said tributary transceivers furthercomprises eleventh logic configured to transmit information using saidsecondary modulation method in response to said poll request.
 6. Thecommunication system of claim 1, wherein said fifth logic is configuredto transmit a poll request as part of said information.
 7. Thecommunication system of claim 6, wherein said program code in said firstone of said tributary transceivers further comprises eleventh logicconfigured to transmit information using said primary modulation methodin response to said poll request.
 8. A master transceiver for use in amultipoint communication system, said communication system including aplurality of tributary transceivers in which a first one of saidtributary transceivers uses a primary modulation method forcommunication and a second one of said tributary transceivers uses asecondary modulation method for communication, said master transceivercomprising: a processor; a storage medium in communication with saidprocessor, said storage medium having program code for exchanginginformation with said tributary transceivers in said multipointcommunication system, said program code comprising: first logicconfigured to establish said primary modulation method for communicationover said communication medium; second logic configured to transmit atraining sequence to notify said first one of said tributarytransceivers of a change from said primary modulation method to saidsecondary modulation method; third logic configured to transmitinformation to and receive information from said second one of saidtributary transceivers using said secondary modulation method; fourthlogic configured to transmit a trailing sequence using said primarymodulation method to notify said first one of said tributarytransceivers of a change from said secondary modulation method to saidprimary modulation method; and fifth logic configured to transmitinformation to and receive information from said first one of saidtributary transceivers using said primary modulation method.
 9. Themaster transceiver of claim 8, wherein said primary modulation method isselected from the group consisting of QAM, CAP, and DMT.
 10. The mastertransceiver of claim 8, wherein said secondary modulation is selectedfrom the group consisting of FSK and PAM.
 11. A tributary transceiverfor use in a multipoint communication system, said communication systemincluding a master transceiver that uses a plurality of modulationmethods for communication, said plurality of modulation methodsincluding a primary modulation method and a secondary modulation method,said tributary receiver comprising: a processor; a storage medium incommunication with said processor, said storage medium having programcode for exchanging information with said master transceiver in saidmultipoint communication system using said primary modulation method,said program code comprising: first logic configured to detect a changefrom said primary modulation method to said secondary modulation method;second logic configured to ignore transmissions on said communicationmedium using said secondary modulation method; and third logicconfigured to detect a trailing sequence transmitted by said mastertransceiver using said primary modulation method after detecting achange from said primary modulation method to said secondary modulationmethod.
 12. The tributary transceiver of claim 11, wherein said primarymodulation method is selected from the group consisting of QAM, CAP, andDMT.
 13. The tributary transceiver of claim 11, wherein said programcode further comprises fourth logic configured to transmit informationto said master transceiver in response to a poll request from saidmaster transceiver.
 14. A method of communication between a mastertransceiver and a plurality of tributary transceivers connected by acommunication medium, a first one of said tributary transceivers using aprimary modulation method for communication and a second one of saidtributary transceivers using a secondary modulation method forcommunication, said method comprising the steps of: said mastertransceiver establishing said primary modulation method as a currentallowed modulation method; said master transceiver transmitting atraining sequence to notify said first one of said tributarytransceivers of a change from said primary modulation method to saidsecondary modulation method as the current allowed modulation method;said first one of said tributary transceivers detecting a change fromsaid primary modulation method to said secondary modulation method asthe current allowed modulation method; said master transceivertransmitting information to said second one of said tributarytransceivers using said secondary modulation method; said first one ofsaid tributary transceivers ignoring transmissions on said communicationmedium using said secondary modulation method; said master transceivertransmitting a trailing sequence using said primary modulation method tonotify said first one of said tributary transceivers of a change fromsaid secondary modulation method to said primary modulation method asthe current allowed modulation method; said first one of said tributarytransceivers detecting said trailing sequence; said master transceivertransmitting information to said first one of said tributarytransceivers using said primary modulation method; and said second oneof said tributary transceivers ignoring transmissions on saidcommunication medium using said primary modulation method.
 15. Themethod of claim 14, wherein said primary modulation method is selectedfrom the group consisting of QAM, CAP, and DMT.
 16. The method of claim14, wherein said secondary modulation method is selected from the groupconsisting of FSK and PAM.
 17. A computer readable medium having aprogram for controlling a master transceiver in a multipointcommunication system, said communication system including a plurality oftributary transceivers in which a first one of said tributarytransceivers uses a primary modulation method for communication and asecond one of said tributary transceivers uses a secondary modulationmethod for communication, said program comprising: first logicconfigured to establish said primary modulation method for communicationover said communication medium; second logic configured to transmit atraining sequence to notify said first one of said tributarytransceivers of a change from said primary modulation method to saidsecondary modulation method; third logic configured to transmitinformation to and receive information from said second one of saidtributary transceivers using said secondary modulation method; fourthlogic configured to transmit a trailing sequence using said primarymodulation method to notify said first one of said tributarytransceivers of a change from said secondary modulation method to saidprimary modulation method; and fifth logic configured to transmitinformation to and receive information from said first one of saidtributary transceivers using said primary modulation method.
 18. Acomputer readable medium having a program for controlling a firsttributary transceiver using a primary modulation method in a multipointcommunication system, said communication system including a secondtributary transceivers using a secondary modulation method forcommunication and a master transceiver, said program comprising: firstlogic configured to detect a change from said primary modulation methodto said secondary modulation method; second logic configured to ignoretransmissions on said communication medium using said secondarymodulation method; and third logic configured to detect a trailingsequence transmitted by said master transceiver using said primarymodulation method after detecting a change from said primary modulationmethod to said secondary modulation method.